Issue on filter when update a job

When update a job, the previous filter definition is merged with the new one instead being rewrited.

To reproduce, create a simple job with the following filter:

    <nodefilters excludeprecedence="true">
      <include>
         <hostname>asuka</hostname>      
      </include>
    </nodefilters>

then, change the filter to:

    <nodefilters excludeprecedence="true">
      <include>
         <name>asuka</name>
      </include>
    </nodefilters>

and after rd-jobs load, the job filter is name=asuka AND hostname=asuka.